The present invention relates to a parallel analog to digital (A/D) converter and a method for processing at least two signals therein, and more particularly, to a sample and hold stage for a parallel A/D converter, and a method for multiplexing at least two analog signals and quantizing each analog input into a set of discrete outputs.
Parallel A/D converters are commonly used in conjunction with video signal processors such as digital television receivers and video recorders. In accordance with super-VHS standards, two signals, luminance and chrominance, must be stored and read off a video tape as separate analog signals. The analog signals must then be converted into digital data for use by the digital receiver. In the past, conversion of these two analog signals has been performed by either of two devices: two parallel A/D converters or a single parallel A/D converter with a multiplexer placed ahead of it. Each system has inherent problems: the dual parallel A/D converter is expensive due to the large number of components required; and the multiplexer/parallel A/D converter distorts the video signal due to its internal capacitance.
The main drawback with employing two parallel A/D converters lies with the high cost of each device. An analog signal is converted into a digital word first by quantizing the signal into discrete outputs, and then by encoding the discrete outputs into a digital word. For an n-bit word, 2.sup.n -1 sample and hold stages are required to quantize the signal. Each sample and hold stage comprises a comparator and a latch. Each additional bit increases the resolution of the output. Therefore, for each additional bit of increased resolution, the required number of comparators and latches doubles. For example, a single-bit parallel A/D converter contains just one comparator and one latch while a seven-bit parallel A/D converter contains 2.sup.7 -1 comparators and as many latches. As the number of latches and comparators doubles, the chip area of a converter implemented as a monolithic integrated circuit increases accordingly The increased size also increases the cost of each A/D converter. Because one A/D converter is assigned to each input signal, doubling the number of analog inputs doubles the cost of A/D conversion
A less expensive means for converting at least two analog signals is to multiplex the analog signals into a single A/D converter. This system also encounters problems: in extreme cases, when the multiplexer switches signals and the input of the A/D converter changes from the positive maximum value of one analog signal to the minimum value of the other analog signal, the charge of the capacitance of the analog input reverses For a seven-bit A/D converter, this input capacitance is somewhere on the order of ten picofarads. Because of the time constant formed by the input capacitance along with the input resistance of the A/D converter and the output resistance of the multiplexer, the signals are distorted. Because distortion increases as the frequency of components of the analog signal increases, the use of a multiplexer ahead of the A/D converter is practically impossible.
Therefore, it is a practical object of the present invention to provide an improved parallel A/D converter and a method for processing two analog signals therein, thereby eliminating the need for a multiplexer ahead of the A/D converter.
A further object of the present invention is to improve the sample and hold stage by multiplexing the analog signals therein such that a possible reversal of charge of the A/D converter's input capacitance is avoided, thereby eliminating distortions caused by the high frequency components of the analog signal.
An additional object of the present invention is to expand the sample and hold stage of a conventional parallel A/D converter such that only three transistors and one additional controlled current path are required for each additional analog input to be processed, thereby adding only a negligible amount of area to the total chip area of the A/D converter.
Another object of the present invention is to provide an A/D converter comprising a plurality of parallel-connected sample and hold stages wherein the additional sample and hold stages increase the resolution of the digital output.
Various other objects and advantages of the present invention shall become clear from the following detailed description of an exemplary embodiment thereof and the novel features will be particularly pointed out in conjunction with the claims appended hereto.